Thermostats are widely used in dwellings and other temperature-controlled spaces. In many cases, thermostats are mounted on a wall or the like to allow for the measurement and control of the temperature, humidity and/or other environmental parameter within the space. Thermostats come in a variety of shapes and with a variety of functions. Some thermostats are electromechanical in nature, and often use a bimetal coil to sense and control the temperature setting, typically by shifting the angle of a mercury bulb switch. These thermostats typically have a mechanical user interface, such as a rotating knob, a slider, or the like, to enable the user to establish a temperature set point. More advanced electronic thermostats have built in electronics, often with solid state sensors, to sense and control various environmental parameters within a space. The user interface of many electronic thermostats includes software controlled buttons and/or a display. It has been found that while electronic thermostats often provide better control, thermostats with a mechanical user interface can be more intuitive for many users. Many users, for example, would be comfortable with a rotating knob that is disposed on a thermostat for setting a desired set point or other parameter.
One factor in operation of HVAC devices such as, for example, a furnace, which is controlled at least partly by the thermostat is the cycle time or cycle rate, or the time between successive startups of the furnace. Cycle time is usually measured not in the actual time between successive startups but instead in terms of the number of startups or cycles per hour, abbreviated “cph.” Thus a cycle time of 20 minutes is the equivalent of 3 cph.
It may be preferred to have a lower cycle rate, typically 3 cph, for high efficiency furnaces for a variety of reasons. Chief among these is the fact that the combustion gasses ejected from a high efficiency furnace are cooled to a level which is very near to the condensing temperature of the water vapor in the combustion gasses. This may cause moisture to condense in the chimney duct and flue during each startup of the furnace. If the cph value is high, the moisture can accumulate because the flue does not get a chance to thoroughly heat and evaporate any condensed moisture. Since these chimney ducts and flues are often at least partly include galvanized steel, accumulated moisture can eventually cause rusting and even perforation of the duct. Perforation of the duct, in particular, is a serious situation since it may allow release of toxic combustion products within living spaces. Less efficient furnaces release combustion gasses at a higher temperature which tends to thoroughly heat and dry out the chimney duct, even with a high cycle rate. It is therefore possible to run less efficient furnaces at higher cycle rates without harm to the flues and ducts. A common cycle rate for furnaces having conventional efficiencies is in the range of 5 cph. Other things being equal (which they often are not), it is preferable to run at a higher cycle rate because room temperature swing during each cycle may be kept smaller at such higher cycle rates. However, when using a higher efficiency furnace, one can often compensate for the larger room temperature swings that result from lower cycle rates by simply increasing the temperature setting slightly for the thermostat.
The present invention provides cost effective methods and apparatus for adjusting cycle rate and/or other parameters of a thermostat using a combination of mechanical and electrical components. Often, the thermostat includes a mechanical user interface, and the mechanical user interface is used to help select a desired parameter value. In some embodiments, an LCD display or the like is not required or even desired, which may help reduce the cost of the thermostat.